bug#29053: 26.0.90: scroller cannot be dragged to bottom of window

[Charles A. Roelli]

With 25.3 under macOS 10.6, from -q:

C-x C-f src/xdisp.c RET

I drag the vertical scroller from its top position (corresponding to
the start of the buffer) down to its lowest (corresponding to end of
buffer), and the mode line says "Bot L33113" and the last line of the
buffer is shown.

In comparison, when doing the same from 26.0.90 (a recent build --
4189d0e, 2017-10-29), the scrolling lags about 2 seconds behind
(possibly another issue), and I cannot bring the scroller to the
bottom of the window -- I only reach "98% L32461".  Actually, the
mouse pointer tracking seems to be off by some vertical pixel offset,
such that if I click and drag the scroller, when the mouse pointer
reaches halfway down the scroll bar, the scroller is not under it, but
only 40% or 45% down.

Other ways to scroll the text up (thus bringing the scroller down
where it cannot be dragged), such as clicking in the area underneath
the scroller, and clicking the scroll bar down arrow, work fine.